Magnetic random access memory (MRAM) devices have been widely used as non-volatile memory devices that may be operated at relatively low voltages and high speeds. In a unit cell of a MRAM device, data may be stored in a magnetic tunnel junction (MTJ) structure of a magnetic resistor. An MTJ structure may include first and second ferromagnetic layers, and a tunneling insulating layer between the first and second ferromagnetic layers. A magnetic polarization of the first ferromagnetic layer (also referred to as a free layer) may be changed using an external magnetic field applied to the MTJ structure. The external magnetic field may be induced by a current flowing adjacent the MTJ structure, and the magnetic polarization of the free layer may be parallel or antiparallel with respect to a fixed magnetic polarization in the second ferromagnetic layer (also referred to as a pinned layer). The current used to generate the external magnetic field may flow through a conductive layer such as a digit line and/or a bit line disposed adjacent the MTJ structure.
According to spintronics based on quantum mechanics, when magnetic spins in the free layer and the pinned layer are arranged parallel with respect to each other, a tunneling current flowing through the MTJ structure may have a relatively high value. When the magnetic spins in the free layer and the pinned layer are arranged antiparallel with respect to each other, a tunneling current flowing through the MTJ structure may have a relatively low value. Therefore, data stored in a MRAM cell may be determined based on a direction of magnetic spins in the free layer.
When viewed from a plan view, MTJ structures may have a rectangular or elliptical shape because the magnetic spins in the free layer may be in a relatively stable state when they are parallel to a longitudinal direction of the free layer.
An integrated circuit MRAM device may include a plurality of MTJ structures. The plurality of MTJ structures may have non-uniform switching characteristics depending on a fabrication process used. In this case, external magnetic fields used to write desired data in the MTJ structures may be different from one memory cell to another. Therefore, a greater non-uniformity of MTJ structure switching characteristics may result in a lower write margin for a MRAM device. In particular, when an MTJ structure is reduced in size to increase integration density, write margins may be significantly reduced. In other words, during a write operation to selectively store desired data in one of the MTJ structures, undesired data may be written to a non-selected MTJ structure(s) that shares a bit line and/or a digit line electrically associated with the selected MTJ structure. That is, according to conventional writing methods, a write disturbance may occur so that undesired data is stored in the non-selected MTJ structure(s) during an operation used to store data in the selected MTJ structure.
In addition, MRAM devices have been proposed which use a spin injection mechanism to reduce write disturbance and to increase integration density. MRAM devices which use a spin injection mechanism are disclosed for example, in U.S. Pat. No. 6,130,814 entitled “Current-Induced Magnetic Switching Device And Memory Including The Same” by Sun. Additional MRAM devices which use a spin injection mechanism are disclosed in U.S. Pat. No. 6,603,677 B2 entitled “Three-Layered Stacked Magnetic Spin Polarization Device With Memory” by Redon et al. The disclosures of U.S. Pat. No. 6,130,814 and U.S. Pat. No. 6,603,677 are hereby incorporated herein in their entirety by reference.
To switch an MRAM cell using the spin injection mechanism, however, high write current density may be required. In this case, an access transistor such as a MOS transistor should provide a current drive capable of generating high writing currents. That is, when the MRAM cell is programmed using a spin injection mechanism, smaller access transistors may be difficult to provide while maintaining a desired current drive.